Integrin adhesion receptors are essential for diverse biological processed such as embryogenesis, hemostasis and metastasis. The most well-studied integrin is the platelet-specific integrin, allbps, which plays an indispensable role in the biological function of platelets. The allbps integrin exists in a predominantly inactive state that is unable to bind ligand. Upon agonist stimulation of platelets, intracellular signals are generated that induce conformational changes within allbps to allow high affinity ligand binding. Precise regulation of ligand binding to allb33 is necessary for maintenance of normal hemostasis. Perturbation of this regulation can lead to pathological conditions such as bleeding or abnormal platelet thrombus formation, contributing to heart attack and stroke. Furthermore, platelet- tumor cell interactions have been implicated in cancer metastasis. As a result, allbps has been considered an important target not only for antithrombotic therapy but also for anti-cancer therapy. However, the current allbps receptor antagonists have limited therapeutic applications and oral administration results in adverse side effects. Therefore, the development of safer and more effective integrin receptor antagonists would provide great therapeutic potential. Recently, increasing attention has focused on proteins that bind to integrin cytoplasmic domains as potential regulators of integrin function. We have shown that CIB1, which binds directly to the cytoplasmic tail of the ccllb subunit, inhibits allbps activation. Therefore, compounds interacting with the CIB1 binding site may potentially act as CIB1 mimetics to inhibit allbpS activation. Furthermore, interaction of talin with the (33 integrin cytoplasmic domain has been shown to induce allbps activation and compounds that inhibit this interaction may also prove to be useful integrin inhibitors. The aims of the current proposal are to 1) modify and optimize our current solid-phase binding assays for HTS and 2) screen small molecule libraries to identify compounds that bind to ocllb within the CIB1 binding site as well as those that inhibit the talin/ps interaction. Follow-up cell-based assays will then be used to test the effectiveness of the identified inhibitors on allbps activation and platelet aggregation. We believe that the discovery of novel intracellular integrin inhibitors will not only lead to the development of therapeutic agents but also to potentially useful tools to study the mechanism of integrin activation.